The present invention relates to newsprint papers having a friction coefficient in an appropriate range and a sufficient sizing quality.
As four-color offset printing has become a mainstream printing process for newsprint papers, it is increasingly important to control the friction coefficient within an appropriate range. Newsprint papers having a high friction coefficient may cause runnability problems such as wrinkles during printing, while those having a low friction coefficient may cause printability problems such as layer-to-layer slippage. It is also important for newsprint papers to show resistance to wetting by fountain solution (sizing quality) to prevent paper breakage, color drift and tension loss caused by the increased number of times that the fountain solution is transferred in four-color printing.
Modern newsprint papers tend to have a higher friction coefficient because the high proportion of deinked pulp means a relatively low proportion of mechanical pulp and therefore smaller amounts of extracts carried from the mechanical pulp, or they contain calcium carbonate as a filler to form neutralized papers.
Conventional methods for decreasing the friction coefficient of paper include internal application of an alkyl ketene dimer (AKD) or talc or external application of a lubricant for coating pigments. However, these methods are not preferable for preparing high quality papers. If AKD is internally applied, AKD migrates within the roll of paper so that the friction coefficient of outer layers of the roll decreases significantly to cause layer-to-layer slippage during printing. If talc is internally applied, the proportion of fillers having a high specific scattering coefficient such as white carbon and calcium carbonate decreases relatively and therefore, the opacity of paper decreases. Lubricants for coating pigments such as polyethylene wax lubricants (see JP-A H03-137295) and styrene lubricants (see JP-A S58-8200) show low dispersion stability in coatings of pH 7.0 or less, and form aggregates called scum under shear during coating, which results in serious operational problems. This problem occurs in newsprint papers, which are typically coated with a coating of pH 7.0 or less containing starch and a surface sizing agent without pigments.
Therefore, it is desirable to develop a chemical agent capable of controlling the friction coefficient of newsprint papers, especially neutralized newsprint papers, within an appropriate range.
On the other hand, the recent prevalence of four-color printing dictates that about 4 times as much fountain solution is loaded as in monochrome printing, with the result that more adhesive materials are released from newsprint papers and deposited on the blanket. Moreover, high DIP levels or neutral papermaking conditions compromise the self-sizing property of base papers to cause an increase in their surface free energy, resulting in an increase in the energy of adhesion between the paper and the blanket (in other words, the energy required for separating paper from the blanket), whereby “neppari” problems are more likely to occur. Water-soluble components contained in newsprint papers are dissolved in a fountain solution used during offset printing. The resultant solution is accumulated on a blanket for use in offset printing. “Neppari” is caused by adhesion or tackiness of the resultant solution, whereby a problem such as bad paper runnability or paper break is encountered.
Newsprint papers can be sized in two ways, i.e. internally and externally. Sizing agents used for internal application to neutralized papers are reactive sizing agents such as alkyl ketene dimers (hereinafter referred to as AKD) and alkenyl succinic anhydrides, but the sizing agents are not wholly retained in paper and unretained sizing agents are hydrolyzed in white water to form deposits, which contaminate paper machines or press rolls. In stock systems containing high levels of mechanical and deinked pulps such as stocks for newsprint papers, a significant amount of anionic trash is included and any cationic emulsified AKD internally added as a sizing agent binds to the anionic trash but not to pulp fibers, and therefore, AKD loadings must be increased to afford an effective sizing quality, and the sizing quality varies with the amount of anionic trash in the stock systems. Moreover, the sizing effect develops too slowly to afford an appropriate sizing quality by internal application of AKD when on-machine sizing is required.
An alternative approach is external application, i.e. application of a surface sizing agent. Surface sizing agents are typically applied to newsprint papers using gate roll coaters capable of forming/transferring films at high speed. However, it is difficult to obtain a sizing effect from polymeric sizing agents known as surface sizing agents such as styrene-maleic copolymers, styrene-acrylic copolymers and olefin-maleic copolymers, because they are designed to be applied to wet paper in the form of a weak coating solution as used with size press coaters, and they afford a sizing effect after drying, and therefore, they cannot sufficiently bind to pulp fibers due to the poor penetration of the coating solution into paper layers even if they are applied with gate roll coaters.
Thus, a method for applying AKD on newsprint papers was proposed (see JP-A H07-119078), but a large amount of AKD must be applied to afford an effective sizing quality because typical newsprint papers made from acidic stocks at pH 4.5-5.5 have a surface pH of 4.5-5.5, which is below the alkalinity required for AKD to produce an appropriate sizing effect, and the sizing effect develops slowly. To control the friction coefficient in an appropriate range, an anti-slip agent had to be applied at the same time.